There is a constant need, in telecommunications, to provide high data transmission rates. This need, together with the demand for ubiquitous connectivity, is exemplified in the context of providing broadband Internet connectivity by satellite. Satellites with multiple spot beam coverage are for example being provided, such as the ViaSat-1 satellite, launched in 2011 and considered one of the highest capacity communications satellites, with a total throughput of about 140 Gbit/s.
To meet this need, changes to the satellite payload architecture and the communication methodology are researched. Transition from single beam to multiple beam systems is an evidence of the former. While advanced payload architectures are being considered, it is also useful to address the transmitter and receiver-based digital processing techniques being used, particularly in view of efficiently using the available electromagnetic spectrum, being a scarce resource.
Some transmitter and receiver-based digital processing techniques are specifically adapted for situations where a single amplifier on board a satellite is used for amplification of multiple satellite links. With the development of wideband amplifiers, joint amplification of multiple carriers allows saving of the payload mass in comparison to the traditional single amplifier per link case. The sharing of satellite payload among many links reduces the hardware costs and complexity, and also provides a degree of flexibility for system designers by making a wide range of configurations available (such as for example the bandwidth of the carriers, the number of carriers, and the carrier spacing) without requiring changes on-board the satellite.
Efficient amplification is, however, typically a non-linear operation and joint amplification of multiple carriers leads to spurious inter-modulation products. This, coupled with the on-board filtering, leads to distortions caused by symbols from other carriers, i.e. adjacent-channel interference (ACI), or from the same carrier itself, i.e. intersymbol interference (ISI). These distortions can cause severe degradation in the link performance, all the more so for higher order (spectrally efficient) modulations. In the absence of compensation techniques, either a large guard-band between the carriers should be used and/or the amplifier should be operated in a linear region. The former results in inefficient frequency carrier segregation, while the latter translates into a power loss compared to single carrier operations depending on the spectral efficiencies of the individual carriers. The benefit of joint amplification in terms of mission costs is more than offset by the loss in power and spectral efficiencies.
To overcome this situation, mitigation techniques are needed, and should be implemented on the ground, to provide upgrade flexibility while keeping the payload complexity intact. The techniques at the transmitter are called “pre-distortion”, whereas those at the receiver are known as “equalization” (EQ). Some on-ground digital processing techniques are used to minimize ISI and ACI effects in a multiple carrier system with high power amplifier (HPA) on board a transparent (or “bent pipe”) satellite. Transparent satellites receive the data-bearing signals from one or more gateways and then redirect them to the ground receivers after performing frequency translation and amplification, as needed.
Single carrier techniques cannot be used in a straightforward manner for multiple carrier techniques, due to the joint processing at the gateway and the absence of adjacent carriers in single carrier techniques. In multicarrier signal digital pre-distortion (DPD) techniques, the compensation technique acts on the superposition of the carriers, prior to transmission on the uplink channel to the satellite. In multicarrier data DPD techniques, the processing occurs at the carrier level, before pulse-shaping, frequency translation, superposition of the carriers, and uplink transmission to the satellite. In both multicarrier signal and data DPD techniques, no change is required to the functions of the satellite transponder and the receivers.
R. Piazza et al, “Multicarrier Digital Pre-distortion/Equalization Techniques for Non-linear Satellite Channels,” Proc. of the 30th AIAA International Communications Satellite Systems Conference (ICSSC-2012), Ottawa, Canada, 24-27 Sep. 2012, discusses the use of DPD and EQ as mitigation techniques in a multiple carrier scenario, i.e. a scenario where multiple carriers are amplified by a single HPA.
N. Kelly et al, “Digital Predistortion Feasibility Studies for Multicarrier Satellite Communication Systems,” Proc. of the 31th AIAA International Communications Satellite Systems Conference (ICSSC-2013), Florence, Italy, Oct. 14-17, 2013, discusses the applicability of DPD techniques to the linearization of multicarrier satellite communication systems. The paper focuses on a signal DPD technique, involving pre-distortion applied after combining the multiple carriers into a single multicarrier signal.
U.S. Pat. No. 5,598,436 relates to the distortion of a modulation constellation in a monocarrier system (col. 1, lines 64-67) and the distortion of a single OFDM multicarrier signal in a multicarrier system (col 2, lines 30-33; FIGS. 9 and 11).
It is desirable to improve the methods of the prior art, with in mind notably the system performance, processing requirements, complexity and robustness to noise.